(1) Field of the Invention
The present invention relates to a communication system containing a master station and a plurality of slave stations wherein communication between the master station and the plurality of slave stations is carried out by polling.
(2) Description of the Related Art
In a conventional communication system containing a master station and a plurality of slave stations which are usually located so far from the master station that an unignorable (round trip) delay exists in signal transmission between each slave station and the master station, the master station gives equal allowance to communicate with the master station to each slave station in a simple cyclic order by polling.
FIG. 1 shows an arrangement of a communication system between a master station and a plurality of slave stations.
The master station A and a plurality of slave stations B.sub.1, B.sub.2, . . . B.sub.n-1, B.sub.n are connected by transmission lines 101 and 102 each for transmitting signals in directions from the master station A to each slave station and from each slave station to the master station, respectively.
In the above arrangement, the master station has a polling table as shown in FIG. 2, which points to the addresses of the slave stations in a cyclic order, and each address which is pointed to is renewed after each polling operation. The master station polls (gives an allowance to communicate with the master station) to equally to each slave station in a simple cyclic order using the polling table.
The cycle of the above polling is determined so that a communication with each slave station can be completed without interference with a communication with another slave station
In the conventional communication system in the arrangement of FIG. 1, it is not considered whether or not each slave station has a request for communication with the master station, or how much data is requested to be transmitted between the master station and each slave station at the moment of polling.
Therefore, the polling is equally carried out for each slave station regardless of the status of requests as above. The efficiency of communications between the master station and the plurality of slave stations is low due to the polling of a slave station which does not have a request for communication with the master station, and a possible occurrence of waiting for an allowance of communication with the master station, in a slave station.